Infertility can result from defects in the normal process of folliculogenesis. This is seen in the short luteal phase, hyperandrogenic ovaries and premature ovarian failure. Insight into the cause of such abnormal activity can best be obtained by first identifying the factors which regulate the early stages of follicle growth. This is the likely site where the improper function initiates. Access to human ovarian tissue and the extent of insult necessary to obtain such information is not feasible in the human female. Therefore these questions are best pursued in nonhuman primates where morphological and endocrine profiles mimic the hypothalamic-pituitary-ovarian axes of the human and the whole ovary can be evaluated. The hormones most likely to trigger early follicle growth in these species are estrogen and follicle stimulating hormone (FSH). Recent studies by the P.I. in monkeys have shown that preantral follicles increase in size and number during the periovulatory period when there is elevated local estrogen and FSH. They also revealed: that small antral follicles are the pool from which the dominant follicle is most likely recruited in a given menstrual cycle, and follicles of that size are probably controlled by FSH. In order to determine how estrogen alone and estrogen augmentation of FSH affects the early stages of follicle growth, the premenarchial monkey (11-15 month of age) will be used as the model. At this age these primates have ovaries that are sensitive to these hormones but their existing endogenous gonadotropin stimulation is insufficient to support cyclic ovarian function culminating in ovulation. During the experimental interval, hormonal profiles will be monitored and this data will be correlated with the changes in numbers and sizes of preantral and early antral follicles in each ovary. The data will reveal how the presence of local estrogen alone or followed by challenges of FSH will: 1. influence early follicle growth and; 2. at what stages of development FSH receptors can first be identified morphologically.